1. Field of the Invention
The present invention concerns integrated circuits with memory, comprising a fraud-prevention device.
2. Description of the Prior Art
Many integrated circuits with memories have non-volatile memories of the EPROM type, that is, erasable programmable read-only memories that are erasable by exposure to ultra-violet radiation, or of the EEPROM (or electrically erasable programmable read-only memory) type.
The present trend is towards the increasing use of EEPROMs because of their advantages over EPROMs. These advantages include, especially, the fact that the erasure is done electrically, the programming time of these memories which is ten times shorter than for EPROM memory cells, the fact that their programming current is zero and also the possibility they offer of using a programming voltage generated inside the circuit from the Vcc supply voltage of the memory cells.
EEPROM cells are programmed by a programming voltage Vpp having a value of about 20 volts. Although the programming voltage used is 20 volts, the EEPROM cells are programmed at voltages which are below 20 volts and may reach 10 volts. Using a voltage of 20 volts ensures a level acceptable to the threshold voltage V.sub.t of each memory cell.
Now, it so happens that CMOS technology is used in most integrated circuits. As compared with NMOS technology, CMOS technology enables a considerable reduction in static consumption and an increase in speed and in the range of the supply voltage at which the circuit works.
In practice, the operating range of CMOS circuits is from 2V to 15V. The supply voltage of circuits of this type may therefore range from 2 volts to 15 volts. It can be seen that voltages at these levels are of a magnitude sufficient for programming or are even greater than this programming threshold voltage. Now, the use of supply voltages close to or greater than this programming threshold voltage damages the information contained in the memory. Supply voltages at this level are excessive.
A fraudulent person might therefore think of using this phenomenon, during the reading of memory cells, to alter the state of the information contained in these cells by increasing the supply voltage so as to destroy information, inconvenient to him, such as confidential codes or information concerning a bank account.
The invention can be used to resolve this problem, not by preventing the supply voltage from reaching the programming threshold beyond which the reading operation is converted into an operation for writing the memory partially or entirely but, in an unexpected way, by providing an additional or a warning cell in the memory connected to a sensor comprising a threshold comparator which permanently receives information on the state of the warning cell, with the output signal of the sensor either turning off the internal clock of the circuit or triggering an interruption in the internal logic so as to protect the memory as soon as there is a change in the detected voltage.